1. Field of the Invention
This invention relates to conductive elements for antennas and, more particularly, to a conductive element allowing improved log-periodic dipole array performance.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Log-periodic dipole array (LPDA) antennas are popular broadband antennas for many applications. An LPDA includes an array of electric dipoles having varying length extending outward from a pair of feed conductors. The pairs of elements are arranged from shortest to longest, with both the element length and the spacing between elements varying logarithmically along the antenna. The LPDA is a type of “quasi-frequency-independent” antenna, having relatively constant radiation pattern and input impedance characteristics over a frequency range extending (approximately) from the half-wavelength frequency of the longest dipole to the half-wavelength frequency of the shortest dipole.
The LPDA is typically oriented during use such that the end with the shortest elements is pointed in the desired direction of transmission or reception. Furthermore, the antenna is generally designed to be fed at the end with the short elements. These practices help to avoid pattern distortions by reducing effects such as shadowing, reflections, and excitation of harmonics in the longer elements. The feeding at the front end (the short-element end) of the antenna is typically accomplished by running a coaxial feed line along the interior of one of the conductors to which the antenna elements are connected. In this way, the feed signal can be brought to the front of the antenna, while the connector to the signal source (or receiver) is at the back (the long-element end). In such an arrangement, the inner conductor of the coaxial feed line is kept isolated from the outside of the conductor through which it is fed, and connected to the other conductor, so that the feed voltage is applied across the two conductors. An illustration of this connection at the front of an LPDA is shown in FIG. 1. In this embodiment, feed line inner conductor 16 is isolated from outer conductor 12 by insulator 18. Inner conductor 16 is connected to conductor 14, which is in this case a solid conductor, so that the feed voltage may be dropped between conductor 14 and conductor 12.
In addition to the mechanical convenience generally realized by having the connector at the back of the antenna, and the reduced possibility of pattern interference from having a connector at the front, the arrangement of FIG. 1 provides the advantage of creating an intrinsic balancing mechanism for the antenna. Connection of a typical single-ended, or unbalanced, feed voltage directly across the front end of the antenna, on the other hand, would require use of an additional balancing transformer. In the particular configuration of FIG. 1, the inner surface of conductor 12 functions as the shield of the coaxial feed line. At the end of the conductor, currents induced in the shield may flow back along the outer surface of conductor 12, resulting in a balanced line. (The antenna currents described herein are AC currents and exist only within a few skin depths of the surface of a conductor. AC current can therefore flow in one direction on one surface of a conductor, such as the inner wall of a tube, and in the other direction on another surface of the conductor, such as the outer wall of the tube. The tube wall is so many skin depths thick that the current on the inner wall doesn't “see” the current on the outer wall.) In some practical LPDA configurations, as discussed further below, the feed conductor is separate from the coaxial feed line shield. In such cases the feed line shield is connected to the conductor to allow the return current path.
In order for the antenna's radiation to be directed “forward” (out from the short-element end), even though it is being fed “backwards” (feed signal starting at short-element end and traveling along transmission line toward long-element end), the phasing of the feed signals seen by each dipole must be such that the radiation adds constructively in the reverse direction to that of the feed signal travel. In particular, alternating pairs of elements must be fed by signals 180° out of phase. Referring to FIG. 1, elements 20a and 20b constitute the first, and smallest, pair of dipole elements in the LPDA, while elements 22a and 22b constitute the next pair. Several other element pairs not shown in FIG. 1 would typically be present, extending from points further along the feed conductors. It can be seen that for the first element pair in FIG. 1, the upper element is connected to conductor 14, and the lower element to conductor 12. For the second element pair, the opposite is true: the upper element is connected to conductor 12, and the lower to conductor 14. The feed voltage applied between the upper and lower halves of the first dipole, therefore, is of opposite polarity to the feed voltage applied between the corresponding halves of the second dipole. The third dipole (not shown) in such an arrangement would be connected in the manner of the first, the fourth in the manner of the second, and so on.
Because each feed conductor in the LPDA of FIG. 1 has elements extending from it in two directions, the two feed conductors cannot be arranged side-by-side in the plane of the elements. Instead, feed conductors 12 and 14 are spaced apart within a plane perpendicular to that of the elements. This arrangement leads to an offset in position between the halves of the dipole, as represented by distance “D” between the positions of elements 20a and 20b in FIG. 1. Ideal dipoles have both of their elements arranged along the same line, and the offset of FIG. 1 can give rise to cross-polarization and pattern distortion. It would therefore be desirable to minimize this offset by, for example, minimizing the spacing between conductors 12 and 14.
The spacing between the conductors is constrained by other design considerations, however. In fact, the conductor spacing affects the antenna performance more directly and strongly in other ways than through the cross-polarization distortion described above. As alluded to above and shown in FIG. 1, the combination of conductors 12 and 14, and the currents flowing on their outer surfaces, give rise to an overall transmission line 24 feeding the LPDA. The characteristic impedance of this transmission line should be designed such that the input impedance of the antenna matches that of the entire system (including transmitter or receiver) to the extent possible. Furthermore, proper operation of the antenna itself may require the characteristic impedance to have a particular level (or at least lie within a particular range). The characteristic impedance of a transmission line such as line 24 depends upon factors including the spacing between conductors and the shape of each individual conductor. Some “feel” for this can be obtained by considering an approximate expression for the characteristic impedance Z0 of an ideal two-wire transmission line:Z0≈120 ln(2D/d),where D is the conductor-to-conductor spacing and d is the diameter of each of the conductors. In order to reduce the spacing between conductors of the two-wire line without changing the characteristic impedance of the line, therefore, the diameters of the conductors must also be reduced.
The above expression generally does not apply directly to the feed transmission line of an LPDA, however. For example, LPDA conductors are typically not cylindrical as shown in FIG. 1, because such conductors would require brazing or soldering of the elements to the conductors. For improved manufacturability, it is desirable to have conductors with flat surfaces to allow the use of screws to fasten the elements to the conductors. A typical design uses a rectangular tube for a conductor, where the tube has holes formed in a pair of facing walls to allow the elements to be screwed on. A coaxial feed line is then fed through the portion of the tube not blocked by the screws extending through the tube. Feeding the line through in the presence of the screws can be difficult, and reducing the size of the tube (in analogy to reducing the diameter of the conductor in the two-wire line discussed above) would make this assembly more difficult still. Another possible approach to reducing the size of a conductor would be to reduce the diameter of the feed line. For example, various diameters of coaxial cable having a given characteristic impedance are available. Reducing the feed line diameter is not desirable, however, in that a reduced-diameter line has reduced power-handling capability and greater dissipation loss.
It would therefore be desirable to develop a conductor having a shape which allows for close spacing of conductors in an LPDA, while appropriate characteristic impedances are maintained. The desired conductor design should not compromise the performance (such as power-handling capability) or ease of fabrication of the conductor or systems built using it.